1. Field of the Invention
The invention pertains to a system for controlling a piston membrane pump embodied with at least one membrane, whereby the membrane can be moved in a controlled manner as detected by means of indicators and with a desired beat-frequency between two end positions in a pressure chamber to be loaded with hydraulic medium during suction and compression periods (for example, see periods S and P in FIGS. 2 and 3) , and whereby the system is provided with computer means, such as a microprocessor connected to the indicators, for the purpose of providing the computer means with position-indication signals, and a loading and unloading device connected to the computer means and the pressure chamber, for the purpose of adding and removing hydraulic medium during respective loading and unloading periods (for example, see periods c and d in FIGS. 2 and 3).
2. State of the Art
A prior art system is shown in U.S. Pat. No. 4,966,528, for use in pumping aggressive or abrasive media. The piston membrane pump included in the known system possesses an elastically moveable membrane which separates moving parts of the pump from the medium to be pumped. The pumping motion of the membrane is brought about by using moving parts such as a piston, piston arm, etc., to periodically remove and add hydraulic medium from and to the pressure chamber during respective suction and compression periods. Electrical indicators, embodied as displacement detectors, are mounted on an arm fixed to the membrane, and these furnish a signal which is used as a basis for detecting the end positions in a range of movement of the membrane. The end positions, between which the membrane should move if the possibility of fracture is to be as small as possible, delimit the suction and compression periods occurring during the movement of membrane between end positions. Since leakage of the hydraulic medium between the cited moving parts of the pump cannot be avoided at the high pressures in the medium required to move the membrane to its end position, the hydraulic medium in the pressure chamber is constantly replenished and pumped off by means of a loading and unloading device under command from the position-indication signals furnished by the indicators. The purpose of this is to keep the pump-yield constant despite the different working pressures and varying viscosity of the sludge to be pumped. Loading of the pressure chamber with hydraulic medium from a reservoir occurs via a controllable filling valve in the loading and unloading device, and happens in the suction period i.e. while there is a relatively low pressure in the pressure chamber; on the other hand, unloading occurs during the pressure period, during an unloading phase which coincides with the pressure period, when there is a high pressure in the pressure chamber.
The known system functions under normal operating conditions with a suction pressure between approximately 1 and 4 bar (atmospheres of pressure), a compression pressure below approximately 120 bar, and an appropriate nominal beat-frequency of the membrane. However, if application occurs under circumstances which deviate substantially from the normal operating conditions, e.g. pumping a medium with a high yield at suction pressures substantially higher than 4 bar or compression pressures higher than 120 bar, the known system appears to be poorly capable of adjusting to the operating conditions concerned and functioning optimally in the working field.
The purpose of the invention is to provide a system for controlling a piston membrane pump, which system has a broader working field and yet functions in an accurate manner, especially when full capacity is attained.
To this end, the system according to the invention is characterized in that it includes computing means having inputs connected to the indicators, and in that the computing means is arranged to use the position-indication signals to calculate the duration of delay periods (as shown for example at e, f in FIG. 3) beginning around those times when the membrane is at its end positions, the calculation occurring on the basis of the chosen length of the respective separate unloading and loading periods (c, d) immediately following the delay periods (e, f).
The advantage of the system according to the invention is that the starting-times of the loading and unloading periods, which coincide with the cessation of the respective delay periods, can be freely chosen, along with the duration of the unloading and loading periods. If so desired, the choice can be made to depend on the specific operating conditions. Above all, the corresponding calculations of the starting-times and durations of the respective delay periods occur in an accurate manner in the computer means. In this manner, the possibility that too much or too little hydraulic medium will be injected into or removed from the pressure chamber during respective loading and unloading periods is substantially reduced, whereby possible overloading of the membrane, particularly at lower beat-frequencies and higher working pressures, is practically excluded, the lifetime of the system is correspondingly increased and wear in the system is reduced. As a result of the high accuracy in controlling the operation of the pump, the regulating system will enable stable and gentle regulation, without the system being subjected to continual filling, unloading, filling, etc.